1. Field of the Invention
Steam turbines whose design includes double shell construction require devices that allow the two shells to expand and contract differentially, without allowing significant leakage out of the steam pipes that carry steam from the outer shell to the inner shell.
2. Description of the Prior Art
A common system employed by turbine-manufacturers is called a bell seal. The bell slides into a tube held by the inner shell providing a minimum radial clearance with the tube, yet allowing vertical differential motion of the inner and outer shells. The bell is also secured to a tube held by the outer shell in such a way that it can slide, permitting differential motion in either the lateral or axial directions relative to the shaft, yet maintaining a small clearance that keeps leakage to a minimum.
After service, this bell seal system is commonly found to have a diametrical clearance between the bell and the inner shell tube of about 0.010 inches. This allows significant leakage and loss of turbine output. Replacement of the bell seal is very expensive and often ineffective, with the clearance and leakage recurring.
The major problem is that the bell seal itself is of very powerful construction and when it becomes hot during starting procedures, while the inner shell tube is still relatively cool, its thermal growth can stretch and crush the opposing surfaces on the inner shell tube. Even during steady state operation, the bell may be hotter than the inner shell tube. The described problem is especially apparent on larger turbines where the bell diameter is greater.
Split rings have been used to seal bell seals and reduce leakage, as described by Stock in U.S. Pat. No. 3,907,308. However, such rings lack the frictional and structural resistance that is essential to prevent vibration in the high frequency, fluid turbulance that exists downstream of turbine valves operating at high pressure. Damage to such split rings has occurred to the degree requiring replacement of the bell seal, using the original design rather than the split ring type devised by Stock.
Smith, et al, in U.S. Pat. No. 2,505,217 uses a circular plate to minimize leakage. However, this approach suffers from the following disadvantages:
1. the circular plate is made to lie in one plane and is not formed as a cylinder; PA1 2. the Smith plate has very little resistance to vibration and would not survive in the steam turbine environment of bell seals; PA1 3. the Smith plate operates in an air environment of low fluid velocity, near atmospheric pressure, not in a high pressure steam flow in the range of 2400 to 3500 psia with steam velocities in the range of 1000 fps (feet per second); and PA1 4. the Smith plate is not threaded to its holder, wherefore it is subject to vibration. PA1 1. it is of relatively thin wall, cylindrical construction to permit yielding without exceeding the elastic limit when thermal expansion causes interference between the tube and the bell seal; PA1 2. it is fabricated from material with a large thermal expansion coefficient, thus permitting clearances for assembly and disassembly, but firm seal contact when hot and running; PA1 3. it is fabricated from material with minimal oxidation tendencies; PA1 4. it is fabricated from material with good ductility at steam turbine initial temperature; and PA1 5. it is threaded to the bell seal to insure against vibration while permitting easy renewal of worn surfaces at minimal cost.
An improvement to the bell seal system that prevents crushing the mating surfaces or stretching them beyond the elastic limit while insuring against vibration would provide significant improvement in turbine efficiency. A further improvement would result from a system that provides for replacement of only a portion of the bell seal, thus reducing costs.